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I’ll take custody of our son, and my wife will take custody of our daughter. That’s fair, isn’t it?

A judge will not want to separate brothers and sisters, preferring instead to keep all siblings together. The bond between siblings should not be broken by a custody decision, especially when these bonds can be very important during the separation and divorce process. Siblings are often strong emotional props for each other throughout the turmoil. Of course there are unique circumstances they may exist, which would temp a judge to deviate from the norm. Those circumstances must be very convincing.

This situation is called "split custody" and still is quite rare. Usually split custody arises when it would be in the best interests of the child(ren).

For example:
When one child has an extremely strong preference for one parent over another and not all the children have this preference, or when the siblings just cannot tolerate each other in a way that is stronger than just normal sibling rivalry.